Receptors in the circulatory system
Receptors continuously monitor conditions within the body, and so they produce a continuous stream of nervous impulses
. The frequency
of these impulses can vary as conditions change.
detect changes in the blood as a result of increased respiration.
Peripheral chemoreceptors are located in the carotid and aortic bodies
(in the carotid arteries on either side of the neck, and in the main artery leaving the heart). In these locations blood might be expected to be well oxygenated.
Central chemoreceptors are located on the surface of the medulla (brainstem) and they monitor the pH of the cerebrospinal fluid (CSF).
Alongside these changes, (rate and depth of) breathing will be affected, causing more efficient release of carbon dioxide and intake of oxygen
Extra carbon dioxide lowers the pH of the blood plasma, as it forms carbonic acid. This sets up a series of events which increase the heart rate so that the blood containing carbon dioxide is more efficiently moved away from the active muscle, and taken to the lungs to allow the carbon dioxide to be exhaled (excreted in breath leaving the body).
At the same time, extra oxygen will be sent round the body to be used in aerobic respiration by the active muscle tissue, as well as reducing any oxygen debt that has been caused by the increased activity.
Impulses from receptors detecting a rise in carbon dioxide (or a fall in oxygen) pass along nerve fibres
to the cardiac centre (cardiovascular centre)
within the medulla oblongata
. This then sends a greater frequency of impulses to the sinoatrial node of the heart. These impulses travel along fibres of the sympathetic nervous system (a section of the autonomic nervous system, itself part of the peripheral nervous system). The 'accelerator nerve
' releases noradrenaline
) at the neuromuscular junction.
- pressure receptors - (also in carotid bodies/arteries/aorta) detect changes in blood pressure as a result of increased heart activity. In a sense they operate in a different way to chemoreceptors. They detect stretching of blood vessels and act to limit the increase in blood pressure. They send impulses to another region of the cardiac centre (cardiovascular centre)
This causes an increased frequency of nerve impulses via the parasympathetic nervous system (another - separate - section of the autonomic nervous system). These pass via a branch of the vagus nerve
[CNX - the 10th cranial nerve], and its normal frequency of impulses slightly inhibits the sinoatrial node - 'the vagal brake' - restricting the resting heart rate to 60-80 (Hz). This 'inhibitory nerve
' releases acetylcholine
at the neuromuscular junction
The sympathetic and parasympathetic nervous systems act in opposition to one another, so stimulation of one inhibits the other.